High concentrations of sulfates in water sources present problems to wetlands and their wildlife inhabitants. An example of great concern is the high level of sulfates entering the Everglades, which is reported to be 60 to 100 times normal background. Sulfates can stimulate microbial sulfate reduction (MSR) wherein sulfate reducing bacteria (SRB) produce sulfide from sulfate in the course of degrading inorganic matter and which controls the methylation and bioaccumulation of neurotoxic methyl mercury (MeHg) in the Everglades. MeHg is a potent neurotoxin that bioaccumulates in fish and other wildlife. Other deleterious effects of high levels of sulfates are the generation of hydrogen sulfide and the accelerated release of nitrogen and phosphorous from soils, termed autoeutrophication.
Acid mine drainage (AMD), sometimes referred to as Acid Rock Drainage, represents a large source of sulfate containing waters. Acid mine drainage (AMD) is low pH water arising from oxidation of iron and other sulfides to sulfuric acid. It is usually considered as water that flows from coal mines or mining waste or tailings, but can occur in metal mining, highway construction and other deep excavations. AMD is a common term sometimes used to refer to any mine operation discharge, many of which are alkaline.
The traditional treatment of AMD is with lime and limestone to neutralize acidity and precipitate out calcium sulfate (gypsum). However, relatively high levels of sulfate remain. Depending on composition and ionic strength, sulfate concentrations of about 1500 mg/l to up to 4000 mg/l, may remain after such treatments. Calcium content is also high due to the lime treatment, and there are other metal ions present as well.
Sulfate removal processes have been the subject of many studies.
A review of sulfate treatment processes are described in Chapter 3 of “Treatment of Sulphate in Mine Effluents”, October 2003, a final report from International Network for Acid Prevention (INAP) Salt Lake City, Utah 84109 USA. Chemical, membrane ion exchange and biological mechanisms are described. The report may be found at
http://www.inap.com.au/public_downloads/Research_Projects/Treatment_of_Sulphate_in_Mine_Effluents_-_Lorax_Report.pdf
U.S. Pat. No. 4,049,513 describes a process wherein lime softener water is brought through a line to an industrial process where dissolved solids are concentrated and sent to a reaction chamber. Calcium hydroxide is added to precipitate sulfate and provide excess calcium hydroxide in the overflow. The overflow is sent to a second reaction chamber where it is mixed with raw water and CO2 or bicarbonate to precipitate calcium carbonate and provide a clear supernatant for the industrial process.
U.S. Pat. No. 6,790,352 describes process where untreated acid mine drainage enters Section I of the process where Mg(OH)2 and high pH effluent from Section II are added. Precipitation of metal hydroxides ensue which are separated as a cake leaving a stream of “demetallized water.” This stream is separated by tangential filtration into a purified water stream and sulfate ladened water. One portion of water from Section I is mixed with Ca(OH)2 and enters Section II where there is produced a calcium sulfate cake and a high pH effluent. The high pH effluent is used as precipitating agent in Section I. The remaining portion of sulfate ladened water is mixed with ammonia to produce ammonia sulfate cake and aqueous ammonia.
Cost effective methods and apparatus are sought to reduce effluent concentrations of sulfate to below 500 mg/l, and more preferably below 250 mg/l. A useful guideline is that the EPA Secondary Drinking Water Regulations recommend a maximum concentration of 250 mg/l for sulfate ions. Many of the water sources generating AMD are located at remote sites, requiring compact and low energy usage systems. Furthermore, waste disposal has to be controlled to prevent despoiling natural resources.
Herein is described an integrated apparatus and process for removal of sulfates from water sources. In a particular use, sulfate is removed from Acid Mine Drainage (AMD) after preliminary lime and/or limestone treatment, or similar initial treatment to reduce sulfate content to below about 10,000 mg/l, more preferably to less than about 5000 mg/l concentration after initial treatment. Milligram per liter (mg/l), or parts per million (ppm) are standard measures of concentration. The process minimizes the use of chemicals by treating, reconcentrating and recycling input species. Capital expenditures are minimized by precipitation of only side streams.